This invention relates to a video signal reproducing apparatus having a field memory for storing one field of a reproduced video signal and, more particularly, to an apparatus for controlling the operation of such field memory.
Video signal reproducing apparatus may include a field memory to obtain stroboscopic still playback, mosaic or solarization and other special effects achievable by storing a field of a reproduced video signal. Currently a field is stored in a write operation of a field memory in which horizontal and vertical synchronization (hereafter referred to as "sync") pulse signals extracted from the reproduced video signal time-synchronize the write operation with the reproduced video signal. However, because with VTR techniques the vertical sync pulse signal is reproduced just after head switching between two playback heads, the vertical sync pulse signal sometimes cannot be obtained. In particular, when the reproduced video signal has a poor S/N ratio or there is a great tracking error the vertical synch pulse signal may not be obtained. As a result, when the tracking error increases to such an extent that a vertical sync pulse signal cannot be obtained, an operator trying to adjust the tracking error cannot see know the extent of the tracking error from a view of the displayed image.
For this reason, an attempt has been made to replace the vertical sync pulse signal with a pseudo vertical sync pulse signal to control the field memory. The pseudo vertical sync pulse signal is obtained from an RF switching pulse signal which is in synchronism with the rotation of the read head drum. The RF switching pulse signal is produced by a pulse generator provided on the read head drum. However, jitter is introduced on the pseudo vertical sync pulse signal due to lack of uniformity of the playback head drum rotation. This causes a phase shift of the pseudo vertical sync pulse signal relative to the horizontal sync pulse signal resulting in the degradation of the displayed image.